Body water and plasma volume shrink when men go to high altitude, in part, because of increased sympathetic-a neural activity and stimulation of the carotid chemoreceptor. In women, the effects of altitude on body water, plasma volume and carotid chemoreceptor activity are likely to be influenced by the hormonal changes of the menstrual cycle. Even at sea level, women have substantial changes in plamsa volume and carotid chemoreceptor activity during the menstrual cycle. Because menses persists at high altitude, hormonal effects on the control of body fluids at altitude are expected. Thus the overall hypothesis of this study is that high altitude acclimatization and hormonal changes during the menstrual cycle affect the regulation of total body water, extracellular fluid volume, and plasma volume. Specifically, we postulate that women who acclimatize to high altitude during the luteal phase of the menstrual cycle will have a lesser reduction in plasma volume than women who acclimatize during the follicular phase as the result of estradiol and progesterone- related alterations in volume regulatory hormones.